24 V DC circuit example
Given: I = 10 A, L = 15 m, R = 3.08 Ω/km, supply = 24 V
Working: Vdrop = 2 × 10 × 15 × 3.08 / 1000 = 0.924 V
Voltage drop = 0.924 V, or about 3.85% of 24 V.
Electrical Engineering
Voltage Drop Formula
Learn the voltage drop formula, variables and worked examples, then calculate DC or single-phase resistive voltage drop from current, cable length and conductor resistance.
Governing equation
Vdrop = 2 × I × L × R / 1000
Voltage drop is the reduction in voltage along a conductor caused by current flowing through its impedance. For a simple DC or single-phase resistive circuit, the round-trip conductor resistance is the key input.
Use this form when L is one-way length in metres and R is conductor resistance in ohms per kilometre. For AC circuits with significant reactance or power-factor effects, use the full impedance method.
Live calculator
Quick voltage drop calculator
DC / single-phase resistive approximation using one-way length and conductor resistance.
Result
0.924V
3.85% voltage drop at 24 V supply
Formula variables and units
| Symbol | Meaning | Typical SI unit |
|---|---|---|
| Vdrop | Voltage lost in the cable | V |
| I | Load current | A |
| L | One-way cable length | m |
| R | Conductor resistance | Ω/km |
How to calculate it
- 1Find the load current in amperes.
- 2Measure the one-way conductor length from source to load.
- 3Use the conductor resistance at the relevant material, size and operating temperature.
- 4Multiply by two for the outgoing and return path in a DC or single-phase two-wire circuit.
- 5Divide the voltage drop by supply voltage and multiply by 100 to obtain percentage drop.
Common mistakes
- Using one-way resistance without accounting for the return conductor.
- Mixing metres with resistance values stated in ohms per kilometre.
- Ignoring conductor temperature, which changes resistance.
- Applying the simple resistive formula to long AC feeders where reactance is significant.
Worked examples
230 V single-phase resistive example
Given: I = 16 A, L = 20 m, R = 1.83 Ω/km, supply = 230 V
Working: Vdrop = 2 × 16 × 20 × 1.83 / 1000 = 1.171 V
Voltage drop ≈ 1.17 V, or about 0.51%.
Go deeper
Use the dedicated OneCalcApp tool
The quick calculator above demonstrates the governing equation. Open the related engineering tool for broader inputs, outputs and calculation context where available.
Frequently asked questions
What is the basic voltage drop formula?
For a simple DC or single-phase two-wire resistive circuit, Vdrop = 2 × I × L × R / 1000 when length is in metres and resistance is in ohms per kilometre.
How do I calculate voltage drop percentage?
Divide the calculated voltage drop by the source voltage and multiply by 100.
Why is cable length multiplied by two?
In a two-wire circuit current travels from the source to the load and returns through another conductor, so the resistive path is approximately twice the one-way length.
Does this formula work for three-phase circuits?
Three-phase voltage drop normally uses a square-root-of-three relationship and should include resistance, reactance and power factor when those effects matter.
Related formula guides
Continue with another problem-first engineering equation.